Multilayer coil component and method for manufacturing the same

ABSTRACT

A highly reliable multilayer coil component is provided without forming voids between magnetic ceramic layers and internal conductor layers. According to the multilayer coil component, an internal stress problem is reduced, the direct current resistance is low, and fracture of internal conductors caused by the surge or the like is not likely to occur. An acidic solution is allowed to permeate a magnetic ceramic element from a side surface thereof through a side gap portion which is a region between side portions of the internal conductors and the side surface of the magnetic ceramic element and to reach interfaces between the internal conductors and a magnetic ceramic located therearound. A pore area ratio of the magnetic ceramic of the side gap portion which is located between the side portions of the internal conductors and the side surface of the magnetic ceramic element is set in the range of 6% to 28%.

CROSS REFERENCE TO RELATED APPLICATIONS

Notice: More than one reissue application has been filed for the reissueU.S. Pat. No. 8,004,383. The reissue application numbers are Ser. No.13/973,827 (the present reissue application) filed Aug. 22, 2013 andSer. No. 14/553,613 filed Nov. 25, 2014 (a divisional reissue of U.S.application Ser. No. 13/973,827). The present application is a reissueapplication under 35 U.S.C. §251 of U.S. patent application Ser. No.12/719,564, filed Mar. 8, 2010, now U.S. Pat. No. 8,004,383, issued Aug.23, 2011.

The present application is a continuation of International ApplicationNo. PCT/JP2008/065029, filed Aug. 22, 2008, which claims priority toJapanese Patent Application No. 2007-238624 filed Sep. 14, 2007, theentire contents of each of these applications being incorporated hereinby reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multilayer coil component having thestructure in which a magnetic ceramic element includes a spiral coiltherein, the magnetic ceramic element being formed by firing a ceramiclaminate in which coil-forming internal conductors primarily composed ofAg and magnetic ceramic layers are laminated to each other.

2. Description of the Related Art

In recent years, electronic components have been increasingly requiredto be miniaturized, and also as for coil components, a multilayer typehas been becoming a mainstream.

Incidentally, in a multilayer coil component obtained by simultaneousfiring of a magnetic ceramic and internal conductors, an internal stressgenerated by the difference in coefficient of thermal expansion betweenmagnetic ceramic layers and internal conductor layers degrades magneticcharacteristics of the magnetic ceramic and causes a problem in that theimpedance value of the multilayer coil component decreases and/orfluctuates.

Accordingly, in order to solve the above problem, a multilayer impedanceelement has been proposed in which voids are formed between magneticceramic layers and internal conductor layers by a treatment to immerse afired magnetic ceramic element in an acidic plating solution so as toavoid the influence of stress by the internal conductor layers to themagnetic ceramic layers and to overcome the decrease and/or fluctuationof the impedance value, as disclosed in Japanese Unexamined PatentApplication Publication No. 2004-22798.

However, in the multilayer impedance element disclosed in JapaneseUnexamined Patent Application Publication No. 2004-22798, sincediscontinuous voids are formed between the magnetic ceramic layers andthe internal conductor layers by immersing the magnetic ceramic elementin the plating solution so as to enable the plating solution to permeatethe inside of the magnetic ceramic element through portions of theinternal conductor layers which are exposed on the surfaces of themagnetic ceramic element, the internal conductor layers and the voidsare formed between the magnetic ceramic layers, and the internalconductor layers are thinned, so that in practice, the ratio of theinternal conductor layers present between the ceramic layers inevitablydecreases.

Hence, a problem may arise in that a product having a low direct currentresistance is difficult to obtain. In particular, in the case of acompact product, such as a product having dimensions of 1.0 mm, 0 5 mm,and 0.5 mm or a product having dimensions of 0.6 mm, 0.3 mm, and 0.3 mm,the thickness of each magnetic ceramic layer must be decreased, andinternal conductor layers each having a large thickness are difficult toform while the internal conductor layers and the voids are both providedbetween the magnetic ceramic layers. Accordingly, the direct currentresistance is not only decreased but also fracture of the internalconductors caused by the surge or the like is liable to occur, and as aresult, a problem in that sufficient reliability cannot be ensured mayoccur.

SUMMARY OF THE INVENTION

The present invention has been conceived to solve the problems describedabove, and an object of the present invention is to provide a highlyreliable multilayer coil component in which without forming voids as inthe past between magnetic ceramic layers and internal conductor layers,both of which form a multilayer coil component, internal stressesdisadvantageously generated between the magnetic ceramic layers and theinternal conductor layers due to the differences in sintering shrinkagebehavior and coefficient of thermal expansion therebetween can bereduced; the direct current resistance is low; and fracture of theinternal conductors caused by the surge or the like is not likely tooccur.

In order to achieve the above object, in an embodiment of the presentinvention, a multilayer coil component includes: a magnetic ceramicelement formed by firing a ceramic laminate which is formed bylaminating magnetic ceramic layers to each other and which includescoil-forming internal conductors primarily composed of Ag. The internalconductors are interlayer-connected to each other to form a spiral coil.No voids are present at interfaces between the internal conductors and amagnetic ceramic located therearound. The internal conductors areseparated from the magnetic ceramic at the interfaces therebetween.

In the multilayer coil component of the present invention, in a side gapportion between side portions of the internal conductors and acorresponding side surface of the magnetic ceramic element, a pore arearatio of the magnetic ceramic is preferably set in the range of 6% to20%.

The pore area ratio of the magnetic ceramic of the side gap portion ispreferably set larger than the pore area ratio of an external layerregion between an upper surface of the uppermost external layer of theinternal conductors in the magnetic ceramic element and an upper surfacethereof and the pore area ratio of an external layer region between alower surface of the lowermost external layer of the internal conductorsin the magnetic ceramic element and a lower surface thereof.

As the magnetic ceramic, a ceramic which includes NiCuZn ferrite as aprimary component and which contains 0.1 to 0.5 percent by weight of azinc borosilicate-based low softening point glass having a softeningpoint of 500° C. to 700° C. is preferably used, and furthermore, aceramic containing 0.2 to 0.4 percent by weight of the zincborosilicate-based low softening point glass is more preferably used.

In addition, as the magnetic ceramic, a ceramic further containing 0.3to 1.0 percent by weight of SnO₂ is preferably used, and furthermore, aceramic containing 0.5 to 0.8 percent by weight of SnO₂ is morepreferably used.

In addition, the average value of the diameters of pores relating to thepore area ratio of the magnetic ceramic is preferably in the range of0.1 to 0.6 μm.

In addition, another embodiment of the present invention is directed amethod for manufacturing a multilayer coil component. The methodincludes: a step of forming a magnetic ceramic element by firing aceramic laminate in which magnetic ceramic layers and coil-forminginternal conductors primarily composed of Ag are laminated to eachother, the magnetic ceramic element including a spiral coil therein; anda step of allowing an acidic solution to permeate the magnetic ceramicelement from a side surface thereof through a side gap portion which isa region between side portions of the internal conductors and the sidesurface of the magnetic ceramic element and to reach interfaces betweenthe internal conductors and a magnetic ceramic located therearound so asto cut bonds between the internal conductors and the magnetic ceramiclocated therearound at the interfaces therebetween.

In addition, in another embodiment, a method for manufacturing amultilayer coil component of the present invention includes: a step offiring a ceramic laminate including magnetic ceramic green sheetslaminated to each other and coil-forming internal conductor patternsprimarily composed of Ag to form a magnetic ceramic element whichincludes a spiral coil therein, which has two side surfaces facing eachother on which two end portions of the spiral coil are exposed, andwhich has a side gap portion having a pore area ratio of 6% to 20%, theside gap portion being a region between side portions of the internalconductors and a corresponding side surface of the magnetic ceramicelement; a step of forming external electrodes on the two side surfacesof the magnetic ceramic element on which the two end portions of thespiral coil are exposed; and a step of performing plating on thesurfaces of the external electrodes using an acidic plating solution.

In the multilayer coil component of the present invention, which is amultilayer coil component formed by firing a ceramic laminate in whichmagnetic ceramic layers and coil-forming internal conductors primarilycomposed of Ag are laminated to each other, since no voids are presentat the interfaces between the internal conductors primarily composed ofAg and the magnetic ceramic located therearound, and the internalconductors are separated from the magnetic ceramic at the interfacestherebetween, without providing voids at the interfaces between theinternal conductors and the magnetic ceramic (i.e., without thinningeach internal conductor), stress relaxation can be performed. Hence, ahighly reliable multilayer coil component can be provided in which thevariation in characteristics is small, the direct current resistance canbe reduced, and fracture of the internal conductors caused by the surgeor the like can be suppressed or prevented.

In addition, in the case in which the pore area ratio of the magneticceramic in the side gap portion which is the region between the sideportions of the internal conductors and the side surface of the magneticceramic element is set in the range of 6% to 20%, even when aferrite-based ceramic capable of realizing a high strength and a highmagnetic permeability as the entire multilayer coil component is used asthe magnetic ceramic, an acidic solution is allowed to efficientlypermeate the magnetic ceramic element, and without providing voids atthe interfaces between the internal conductor layers and the magneticceramic, the bonds therebetween can be cut at the interfaces.

In addition, when the pore area ratio of the magnetic ceramic in theside gap portion is set large than the pore area ratio of the externallayer region between the upper surface of the uppermost external layerof the internal conductors in the magnetic ceramic element and the uppersurface thereof and the pore area ratio of the external layer regionbetween the lower surface of the lowermost external layer of theinternal conductors in the magnetic ceramic element and the lowersurface thereof, an acidic solution is allowed to efficiently permeatethe magnetic ceramic element through the side gap portion.

In addition, since the external layer region has a small pore arearatio, a multilayer coil component having a desired strength as a wholecan be obtained.

In addition, since the ceramic which includes NiCuZn ferrite as aprimary component and which contains 0.1 to 0.5 percent by weight of azinc borosilicate-based low softening point glass having a softeningpoint of 500° C. to 700° C. is used as the magnetic ceramic, even whenthe magnetic ceramic includes pores and has a low density, a multilayerinductor having a high strength and a high magnetic permeability as theentire multilayer coil component can be obtained.

In addition, since the zinc borosilicate-based low softening point glassis a crystallized glass, a sintered density of the magnetic ceramic canbe stabilized. Furthermore, when the ceramic containing 0.2 to 0.4percent by weight of the zinc borosilicate-based low softening pointglass is used, the above effect can be further improved.

In addition, as the magnetic ceramic, when the ceramic is used whichincludes NiCuZn ferrite as a primary component, which contains a zincborosilicate-based low softening point glass in the amount describedabove, and also which contains 0.3 to 1.0 percent by weight of SnO₂, amultilayer coil component can be obtained which has superior externalstress resistance and direct current superposition characteristics.

In addition, when the ceramic containing 0.5 to 0.8 percent by weight ofSnO₂ is used, the effect described above can be further ensured.

In addition, when SnO₂ is added, the magnetic permeability of themagnetic ceramic is decreased, and the strength is also decreased.However, when the zinc borosilicate-based low softening pointcrystallized glass is added, the decreases in magnetic permeability andstrength can be compensated for.

In addition, according to the present invention, the average value ofthe diameters of pores relating to the pore area ratio of the magneticceramic is preferably set in the range of 0.1 to 0.6 μm, and the reasonsfor this are that when the pore diameter is less than 0.1 μm, an acidicsolution is not likely to reach the interfaces between the internalconductors and the magnetic ceramic located therearound through the sidegap portion, and when the pore diameter is more than 0.6 μm, thestrength of the magnetic ceramic element is decreased.

In addition, in the method for manufacturing a multilayer coil componentof the present invention, since an acidic solution is allowed topermeate the magnetic ceramic element from the side surface thereofthrough the side gap portion and to reach the interfaces between theinternal conductors and the magnetic ceramic located therearound so asto cut the bonds between the internal conductors and the magneticceramic located therearound at the interfaces therebetween, even whenthe end surfaces of the magnetic ceramic element are covered with theexternal electrodes, the acidic solution can reliably reach theinterfaces between the internal conductors and the magnetic ceramiclocated therearound by permeation, and hence stresses at the interfacesbetween the internal conductors and the magnetic ceramic locatedtherearound can be reduced. As a result, a highly reliable multilayercoil component can be manufactured in which the variation incharacteristics is small, the direct current resistance can be reduced,and fracture of the internal conductors caused by the surge or the likeis not likely to occur.

In addition, according to the method for manufacturing a multilayer coilcomponent of the present invention, the magnetic ceramic element isformed which includes a spiral coil therein, which has two side surfacesfacing each other on which respective two end portions of the spiralcoil are exposed, and which has a side gap portion having a pore arearatio of 6% to 20%, and after the external electrodes are formed on thetwo side surfaces of the magnetic ceramic element on which the two endportions of the spiral coil are exposed, plating is performed on thesurfaces of the external electrodes using an acidic plating solution.Hence, even when the end surfaces of the magnetic ceramic element arecovered with the external electrodes, the plating solution (acidicsolution) can reliably reach the interfaces between the internalconductors and the magnetic ceramic located therearound by permeationthrough the porous side gap portion having a pore area ratio of 6% to20% to cut the bonds between the internal conductors and the magneticceramic located therearound at the interfaces therebetween, so that thestress applied to the magnetic ceramic can be reduced.

In addition, since an acidic solution is used as the plating solution,and the plating solution is allowed to permeate the magnetic ceramicelement simultaneously when plating is performed, a new step is notnecessarily added to the existing steps, and hence a highly reliablemultilayer coil component can be efficiently manufactured.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a front cross-sectional view showing the structure of amultilayer coil component according to an example (e.g., Example 1) ofthe present invention.

FIG. 2 is an exploded perspective view showing an important structure ofthe multilayer coil component according to Example 1 of the presentinvention.

FIG. 3 is a side cross-sectional view showing the structure of themultilayer coil component according to Example 1 of the presentinvention.

FIG. 4 is a view illustrating a measurement method of a pore area ratioof a multilayer coil component which is performed in Example 1 of thepresent invention and in a comparative example.

FIG. 5 is a view showing a SIM image of a surface (W-T surface)processed by FIB after a cross section of the multilayer coil component(e.g., sample of Sample No. 3 in Table 1) according to Example 1 of thepresent invention.

FIG. 6 is a view showing a SEM image of a fracture surface of themultilayer coil component (sample of Sample No. 3 in Table 1) accordingto Example 1 of the present invention which is obtained by a three-pointbending test.

FIG. 7 is a view showing the relationship between the impedance and thesoftening point of a zinc borosilicate-based low softening point glassadded to a magnetic ceramic.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, with reference to examples of the present invention, thefeatures of the present invention will be described in more detail.

Example 1

FIG. 1 is a cross-sectional view showing the structure of a multilayercoil component (e.g., multilayer impedance element in Example 1)according to one example of the present invention, and FIG. 2 is anexploded perspective view showing a manufacturing method of themultilayer coil component. This multilayer coil component 10 ismanufactured through a step of firing a laminate 3 in which coil-forminginternal conductors 2 primarily composed of Ag and magnetic ceramiclayers 1 are laminated to each other, and a magnetic ceramic element 3includes a spiral coil 4 therein.

In addition, a pair of external electrodes 5a and 5b is provided at twoend portions of the magnetic ceramic element 3 so as to be electricallyconnected to two end portions 4a and 4b of the spiral coil 4,respectively.

In addition, in this multilayer coil component 10, as schematicallyshown in FIG. 1, no voids are present at interfaces A between theinternal conductors 2 and a magnetic ceramic 11 located therearound, andthe internal conductors 2 and the magnetic ceramic 11 locatedtherearound are in approximately close contact with each other. However,it is configured that the internal conductors 2 are separated from themagnetic ceramic 11 at the interfaces A therebetween.

In addition, in this multilayer coil component 10, since the internalconductor layers 2 are separated from the magnetic ceramic 11 at theinterfaces A therebetween, voids are not necessarily provided at theinterfaces A in order to cut bonds between the internal conductor layers2 and the magnetic ceramic 11, and without thinning the internalconductors, the multilayer coil component 10 can be obtained in whichthe stress is reduced. Hence, a highly reliable multilayer coilcomponent can be provided in which the variation in characteristics issmall, the direct current resistance can be decreased, and fracture ofthe internal conductors caused by the surge or the like is not likely tooccur.

Next, a method for manufacturing this multilayer coil component 10 willbe described.

(1) A magnetic raw material was prepared in such a way that Fe₂O₃, ZnO,NiO, and CuO were weighed at a ratio of 48.0 mole percent, 29.5 molepercent, 14.5 mole percent, and 8.0 mole percent, and wet mixing wasperformed using a ball mill for 48 hours.

Subsequently, a slurry obtained by the wet mixing was dried by a spraydryer and was calcined at 700° C. for 2 hours.

The calcined material thus obtained was wet-pulverized by a ball millfor 16 hours, and a predetermined amount of a binder was mixed after thepulverization was finished, so that a ceramic slurry was obtained.

Next, this ceramic slurry was formed into sheets, so that ceramic greensheets each having a thickness of 25 μm were formed.

(2) Next, after via holes were formed in the ceramic green sheets atpredetermined positions, a conductive paste for forming internalconductors was printed on the surfaces of the ceramic green sheets, sothat coil patterns (i.e., internal conductor patterns) were formed.

As the conductive paste, a conductive paste containing 85 percent byweight of Ag was used in which a Ag powder containing 0.1 percent byweight or less of impurity elements, a varnish, and a solvent wereblended together. As the conductive paste for forming coil patterns(i.e., internal conductor patterns), a paste containing Ag at a highcontent, such as a Ag content of 83 to 89 percent by weight, ispreferably used as described above. In addition, when the amount ofimpurities is large, the internal conductor may be corroded by an acidicsolution, and as a result, the direct current resistance maydisadvantageously increase in some cases.

(3) Subsequently, as schematically shown in FIG. 2, after ceramic greensheets 21 on which internal conductor patterns (coil patterns) 22 wereformed were laminated and pressure-bonded to each other, and ceramicgreen sheets 21a on which no coil patterns were formed were furtherlaminated on an upper and a lower surface of the above laminate,pressure bonding was performed at 1,000 kgf/cm², so that a laminate(unfired magnetic ceramic element) 23 was obtained.

This unfired magnetic ceramic element 23 includes therein a laminatetype spiral coil which is formed of the internal conductor patterns(coil patterns) 22 connected by via holes 24. In addition, the number ofturns of the coil was set to 7.5.

(4) Subsequently, after a pressure-bonded block was cut into apredetermined size, debinding was performed, and sintering was performedby changing a firing temperature between 820° C. and 910° C., so that amagnetic ceramic element including the spiral coil therein was obtained.

The sintering shrinkage rate of the magnetic ceramic (ferrite) in firingis 13% to 20%, and that of the internal conductor is 8%. In addition, ina firing temperature range of 820 to 910° C., the sintering shrinkagerate of the internal conductor is approximately constant.

In addition, when it is assumed that the shrinkage rate of the magneticceramic (ferrite) is larger than that of the internal conductor which isthe conductor pattern, that the sintering shrinkage rate of the internalconductor which is the conductor pattern is in the range of 0% to 15%,and that firing is performed at a predetermined temperature, thedistribution of a pore area ratio is generated in the magnetic ceramicelement. As shown in FIG. 3, the pore area ratio of a side gap portion 8which is a region between side portions 2a of the internal conductors 2and a corresponding side surface 3a of the magnetic ceramic element 3 islarger than the pore area ratio of an external layer region 9 between anupper surface of the uppermost external layer of the internal conductors2 in the magnetic ceramic element 3 and an upper surface thereof andthan the pore area ratio of an external layer region 9 between a lowersurface of the lowermost external layer of the internal conductors 2 inthe magnetic ceramic element 3 and a lower surface thereof. That is, theexternal layer region 9 is more densely sintered, and the pores are morefrequently distributed in the side gap portion 8.

As described above, the reason the external layer region 9 is moredensely fired and the pores are more frequently distributed in the sidegap portion 8 is that when the sintering shrinkage rate of the internalconductor 2 is decreased by a predetermined rate as compared to that ofthe magnetic ceramic 11, the difference in sintering shrinkage ratebetween the internal conductor 2 and the magnetic ceramic 11 isgenerated, and the internal conductor 2 suppresses the sinteringshrinkage of the magnetic ceramic 11.

In addition, the sintering shrinkage rate of the internal conductor canbe controlled, for example, by appropriately selecting the content ofthe conductive component (Ag powder) in the conductive paste for forminginternal conductors and the types of varnish and solvent contained inthe conductive paste.

When the sintering shrinkage rate of the internal conductor is less than0%, the internal conductor may not shrink in firing or may expand largerthan that before firing, and it is not preferable since structuraldefects may occur and/or a chip shape may be adversely influenced.

In addition, when the sintering shrinkage rate of the internal conductoris 15% or more, the distribution of the pore ratio is not generated inthe magnetic ceramic element, and while the density of the externallayer region is increased to a predetermined value, a Ni platingsolution cannot permeate the magnetic ceramic element from the side gap.

Hence, the sintering shrinkage rate of the internal conductor ispreferably set in the range of 0% to 15% and is more preferably set inthe range of 5% to 11%.

The measurement of the sintering shrinkage rate of the magnetic ceramicwas performed in such a way that after ceramic green sheets werelaminated to each other and were pressure bonded under the same pressurecondition as that when a multilayer coil component was actuallymanufactured, the laminate thus obtained was cut into a predeterminedsize, followed by firing, and the sintering shrinkage rate was measuredin the lamination direction by a thermal mechanical analyzer (TMA).

In addition, the measurement of the sintering shrinkage rate of theinternal conductor was performed by the following method.

First, after the conductive paste for forming internal conductors wasthinly applied to a glass plate and was then dried, the dried materialwas scraped off and was pulverized using a mortar into a powder.Subsequently, after the powder thus obtained was received in a mold andwas processed by uniaxial press molding under the same pressurecondition as that when a multilayer coil component was actuallymanufactured, cutting was performed to obtain a predetermined size, andfiring was then performed. Next, the sintering shrinkage rate wasmeasured in a direction along the press direction by a TMA.

(5) Subsequently, after a conductive paste for forming externalelectrodes was applied to two end portions of the magnetic ceramicelement (sintered element) 3 including the spiral coil 4 therein and wasdried, firing was performed at 750° C., so that the external electrodes5a and 5b (see FIG. 1) were formed.

Incidentally, as the conductive paste for forming external electrodes, aconductive paste was used in which a Ag powder having an averageparticle diameter of 0.8 μm, a B—Si—K-based glass frit having superiorplating resistance and an average particle diameter of 1.5 μm, avarnish, and a solvent were blended together. In addition, the externalelectrodes formed by firing this conductive paste were dense so as notto be eroded by a plating solution in the following plating step.

(6) Subsequently, the external electrodes 5a and 5b thus formed wereprocessed by Ni plating and Sn plating, so that plating films eachhaving a Ni plating film layer as a lower layer and a Sn plating filmlayer as an upper layer were formed. Accordingly, as shown in FIG. 1,the multilayer coil component (multilayer impedance element) 10 havingthe structure in which the magnetic ceramic element 3 includes thespiral coil 4 therein is obtained.

In addition, in the above plating step, as a Ni plating solution, anacidic solution having a pH of 4 was used which contained nickelsulfate, nickel chloride, and boric acid at a ratio of approximately 300g/L, approximately 50 g/L, and approximately 35 g/L.

In addition, as a Sn plating solution, an acidic solution having a pH of5 was used which contained tin sulfate, ammonium hydrogen citrate, andammonium sulfate at a ratio of approximately 70 g/L, approximately 100g/L, and approximately 100 g/L.

Evaluation Characteristics

For the multilayer coil component formed as described above, measurementof the impedance by the following method and measurement of the bendingstrength by a three-point bending test were performed.

In addition, the pore area ratio of the magnetic ceramic element beforethe external electrodes were processed by plating in the above step (6)was measured by the following method.

(a) Measurement of Impedance

Measurement of the impedance was performed on 50 samples using animpedance analyzer (HP4291A manufactured by Hewlett-Packard Co.), andthe average value was then obtained (n=50 pcs.).

(b) Measurement of Bending Strength

Measurement was performed on 50 samples in accordance with EIAJ-ET-7403,and the strength at a fracture probability of 1% of the Weibull plot wasregarded as the bending strength (n=50 pcs.).

(c) Measurement of Pore Area Ratio

After a cross-sectional surface (hereinafter referred to as “W-Tsurface”) defined by the width direction and the thickness direction ofthe magnetic ceramic element before plating was processed by mirrorpolishing and was then processed by focused ion beam processing (FIBprocessing), the surface thus processed was observed by a scanningelectron microscope (SEM), so that the pore area ratio of the sinteredmagnetic ceramic was measured.

In particular, the pore area ratio was measured using an imageprocessing software “WINROOF” manufactured by Mitani Corporation. Thedetailed measurement method is as follows.

FIB apparatus: FIB200TEM manufactured by FEI

FE-SEM (scanning electron microscope): JSM-7500FA manufactured by JEOLLtd.

WinROOF (image processing software): Ver. 5.6 manufactured by MitaniCorporation

Focused Ion Beam processing (FIB processing)

As shown in FIG. 4, FIB processing was performed at an incident angle of5° with respect to a polished surface of a sample which wasmirror-polished by the above method.

Observation by Scanning Electron Microscope (SEM) SEM observation wasperformed under the following conditions.

Acceleration voltage: 15 kV

Sample inclination: 0°

Signal: Secondary electrons

Coating: Pt

Magnification: 5,000 times

Calculation of Pore Area Ratio

The pore area ratio was obtained by the following method.

a) The measurement range is determined. When the range is too small,errors caused by measurement points are generated.

(In this Example, the Range was Set to 22.85 μm by 9.44 μm.)

b) When it is difficult to discriminate between the magnetic ceramic andpores, the brightness and the contrast are adjusted. C) The binary imageprocessing is performed so as to extract only pores. When “colorextraction” by the image processing software “WinROOF” is not perfect,manual operation is additionally performed.

d) When images other than pores are extracted, the images other thanpores are eliminated.

e) The total area, the count, the pore area ratio, and the area of themeasurement range are measured by “Measurement of Total Area/Count” ofthe image processing software.

The pore area ratio of the present invention is a value measured asdescribed above.

Table 1 shows the pore area ratios of the side gap portion and theexternal layer region, the impedance (|Z|) value, and the bendingstrength, which were measured as described above. In addition, Table 1also shows the firing temperature, the presence or absence of voids atthe interfaces between the magnetic ceramic and the internal conductorswhich is judged by SEM observation of an FIB-processed surface, and thepresence or absence of separations at the interfaces between themagnetic ceramic and the internal conductors when the multilayer coilcomponent is fractured.

TABLE 1 Impedance Presence of Firing Pore area ratio Pore area ratio |Z|Bending Presence of separations Sample temperature of side gap ofexternal (Ω) strength voids at at No. (° C.) portion (%) layer region(%) 100 MHz (N) interfaces interfaces 1 820 26 20 544 13 NO YES 2 835 2015 595 18 NO YES 3 850 16 12 637 19 NO YES 4 870 11 8 659 20 NO YES 5885 8 5 660 21 NO YES 6 890 6 4 626 21 NO YES 7 910 2 1 373 22 NO NO

In Table 1, the samples (i.e., samples of Sample Nos. 1 to 6) in each ofwhich no voids are recognized at the interfaces between the magneticceramic and the internal conductors by the SEM observation of the FIBprocessed surface and in each of which the separations are recognized atthe interfaces between the magnetic ceramic and the internal conductorswhen the multilayer coil component is fractured are samples whichsatisfy the requirement of the present invention in which “no voids arepresent at the interfaces between the internal electrodes primarilycomposed of Ag and the magnetic ceramic located therearound, and theinternal conductors and the magnetic ceramic are separated from eachother at the interfaces therebetween”, and Sample No. 7 is a sample inwhich the internal conductors and the magnetic ceramic are bonded toeach other at the interfaces therebetween and is a sample which does notsatisfy the requirement of the present invention.

As described above, as for the sintering shrinkage rate of the magneticceramic (ferrite) and that of the internal conductor in firing, themagnetic ceramic has 13% to 20%, and on the other hand, the internalconductor has 8%; hence, since the sintering shrinkage rate of theinternal conductor is lower than that of the ferrite, at the stage atwhich the firing is finished, the internal conductors and the magneticceramic are tightly bonded to each other at the interfaces therebetween.

However, when a sample in which the internal conductors and the magneticceramic are tightly bonded to each other at the interfaces therebetweenis processed, for example, by Ni plating, and when the pore area ratioof the side gap portion is large to a certain extent, a Ni platingsolution permeates the inside of the magnetic ceramic element(multilayer coil component) from pores in the regions which are notcovered with the external electrodes at the same time when the platingis performed and reaches the interfaces between the internal conductorsand the magnetic ceramic, so that cutting of the bonds between theinternal conductors and the magnetic ceramic at the interfacestherebetween is performed.

On the other hand, when the pore area ratio of the side gap portion issmall, the plating solution cannot permeate the inside, and hence thebonds between the internal conductors and the magnetic ceramic at theinterfaces cannot be cut.

In the case of the sample of sample No. 7 shown in Table 1 in which thepore area ratio of the side gap portion is as low as 2%, and in which noseparations between the magnetic ceramic and the internal conductors arerecognized at the interfaces when the multilayer coil component isfractured, since the internal conductors and the magnetic ceramic arebonded to each other at the interfaces therebetween even after theplating step is performed, and a stress is applied to the magneticceramic due to the sintering shrinkage of the internal conductors, theimpedance is considerably decreased.

On the other hand, in the case of the samples of sample Nos. 1 to 6 inwhich the pore area ratio of the side gap portion is 6% or more, sincethe plating solution permeates the inside of the magnetic ceramicelement, and the bonds between the internal conductors and the magneticceramic at the interfaces therebetween are sufficiently cut, it is foundthat a multilayer coil component having superior characteristics can beobtained in which the decrease in impedance is small.

In addition, in the case of the samples of sample Nos. 1 to 6, althoughno voids are recognized at the interfaces between the magnetic ceramicand the internal conductors by the SEM observation of the FIB processedsurface, when the multilayer coil component is fractured, separationsare recognized between the magnetic ceramic and the internal conductorsat the interfaces therebetween. From the results described above, it isfound that since a Ni plating solution permeates the inside of themagnetic ceramic element (multilayer coil component) from pores in theregions which are not covered with the external electrodes and reachesthe interfaces between the internal conductors and the magnetic ceramic,the bonds between the internal conductors and the magnetic ceramic atthe interfaces therebetween are cut.

In addition, in the case of the sample of sample No. 1, since the porearea ratio is as high as 26%, although the decrease in impedance issmall, the decrease in bending strength is recognized.

Hence, in order to ensure a high bending strength while the decrease inimpedance is suppressed, as in sample Nos. 2 to 6, the pore area ratioof the side gap portion is preferably set in the range of 6 to 20.

In addition, as in sample Nos. 3 to 5, when the pore area ratio is setto 8% to 16%, it is found that more preferably, the impedance and thebending strength are further stabilized.

FIG. 5 shows a SIM image of a surface (W-T surface) processed by FIBafter a cross section of the multilayer coil component (sample of sampleNo. 3 shown in Table 1) according to the example of the presentinvention is mirror-polished.

This SIM image was obtained in such a way that after the W-T surface ofthe multilayer coil component processed by plating was mirror-polishedand was then processed by FIB, observation was performed by a SIM at amagnification of 5,000, and it is found that no voids are recognized atthe interfaces between the magnetic ceramic and the internal conductors.

In addition, FIG. 6 shows a SEM image of a fracture surface of themultilayer coil component (i.e., sample of sample No. 3 shown inTable 1) according to the example which is obtained by a three-pointbending test.

As apparent from FIG. 6, according to the SEM observation of thefracture surface, spaces are recognized, and since the internalconductors and the magnetic ceramic are separated from each other at theinterfaces therebetween, it is believed that when the internal conductorextends by fracture and is pulled to the front side with respect to theplane of the figure, the spaces are formed. In addition, also when thesample is fractured by a nipper, spaces similar to those described aboveare recognized.

Example 2

In Example 2, an example of a multilayer coil component formed using amagnetic ceramic added with a glass will be described.

A magnetic raw material was prepared in such a way that Fe₂O₂, ZnO, NiO,and CuO were weighed at a ratio of 48.0 mole percent, 29.5 mole percent,14.5 mole percent, and 8.0 mole percent, and wet mixing was performedusing a ball mill for 48 hours to form a slurry.

Subsequently, this slurry was dried by a spray dryer and was calcined at700° C. for 2 hours to obtain a calcined material.

Next, after a zinc borosilicate-based low softening point crystallizedglass was added to this calcined material at a ratio of 0 to 0.6 percentby weight and was then wet-pulverized by a ball mill for 16 hours, apredetermined amount of a binder was mixed, so that a ceramic slurry wasobtained. In addition, the zinc borosilicate-based low softening pointcrystallized glass may be added before the calcination.

The zinc borosilicate-based crystallized glass thus added was a glasshaving a composition containing 12 percent by weight of SiO₂, 60 percentby weight of ZnO, and 28 percent by weight of B₂O₃ and was a glasshaving a softening point of 580° C., a crystallization temperature of690° C., and a particle diameter of 1.5 μm.

In addition, as the glass composition, additives, such as BaO, K₂O, CaO,Na₂O, Al₂O₂, SnO₂, SrO, MgO, and the like, may be contained in the abovebasic composition.

Subsequently, this ceramic slurry was formed into sheets, so thatceramic green sheets each having a thickness of 25 μm were obtained.

Next, by the same method as that including the steps (2) to (4) ofExample 1, an unfired laminate (magnetic ceramic element) including alaminate type spiral coil therein was formed.

In addition, this laminate was sintered by adjusting the firingtemperature so as to obtain a pore area ratio of the side gap portion of11%.

Next, by the same method and conditions as those of Example 1, theimpedance and the bending strength by a three-point bending test weremeasured.

In Table 2, the values of impedances (|Z|) and the values of bendingstrengths of samples which used magnetic ceramics containing differentamounts of the glass are shown.

TABLE 2 Glass addition Impedance |Z| Bending Sample amount (percent (Ω)strength No. by weight) 100 MHz (N)  8 0 659 20  9 0.05 661 21 10 0.10665 24 11 0.20 679 25 12 0.30 681 26 13 0.40 676 26 14 0.50 665 25 150.60 645 24

As shown in Table 2, by addition of the zinc borosilicate-basedcrystallized glass, even having a predetermined pore area ratio and alow density, a magnetic ceramic can be obtained which has a highmechanical strength and a high magnetic permeability. Accordingly,without decreasing the impedance, a multilayer coil component having ahigh bending strength can be obtained.

In addition, the addition amount of the zinc borosilicate-basedcrystallized glass is preferably set in the range of 0.1 to 0.5 percentby weight and is more preferably set in the range of 0.2 to 0.4 percentby weight.

In addition, the composition of the zinc borosilicate-based crystallizedglass used in Example 2 was changed, and a zinc boro silicate-basedcrystallized glass having a softening point of 400° C. to 770° C. wasformed. In addition, a multilayer coil component was formed by the samemethod and conditions as those of Example 1 except that the additionamount of this zinc borosilicate-based crystallized glass was set to 0.3percent by weight, and the impedance of the multilayer coil componentthus obtained was measured. The results are shown in FIG. 7.

As can be seen from FIG. 7, when the softening point of the glass to beused is set in the range of 500° C. to 700° C., a high impedance (|Z|)value can be obtained.

When the glass softening point is less than 500° C., it is notpreferable since the sintering of the magnetic ceramic is disturbed dueto a decrease in fluidity and the magnetic permeability is decreased dueto evaporation of the glass.

In addition, when the glass softening point is more than 700° C., it isalso not preferable since the sintering of the magnetic ceramic isdisturbed, the magnetic permeability is decreased, and the impedance isdecreased.

In addition, in the present invention, a method for controlling the porearea ratio of the side gap is not particularly limited, and for example,when the following methods are used alone or in combination, the porearea ratio of the side gap can be controlled. That is, for example,there may be mentioned:

a method (1) for adjusting the difference in sintering shrinkage ratebetween the magnetic ceramic and the internal conductor within the rangeof 5% to 20%;

a method (2) for adjusting the thickness of the internal conductor tothe thickness of a magnetic ceramic sheet (such as 10 to 50 μm), forexample, within the range of 5 to 50 μm;

a method (3) for adjusting the particle diameter of a ceramic formingthe magnetic ceramic sheet, for example, within the range of 0.5 to 5μm;

a method (4) for adjusting the content of a binder of the magneticceramic sheet, for example, within the range of 8 to 15 percent byweight; and

a method (5) performed using the above methods (1) to (4) incombination.

Example 3

In Example 3, an example of a multilayer coil component formed using amagnetic ceramic in which SnO₂ was added to NiCuZn ferrite will bedescribed.

After Fe₂O₃, ZnO, NiO, and CuO were weighed at a ratio of 48.0 molepercent, 29.5 mole percent, 14.5 mole percent, and 8.0 mole percent, SnOwas weighed at a ratio of 0 to 1.25 percent by weight to the primarycomponents (that is, at a ratio of 0 to 1.2 percent by weight of thetotal weight) to form a magnetic raw material, and wet mixing wasperformed using a ball mill for 48 hours, so that a slurry was formed.

Subsequently, this slurry was dried by a spray dryer and was calcined at700° C. for 2 hours to obtain a calcined material.

After 0.3 percent by weight of a zinc borosilicate-based low softeningpoint crystallized glass was added to this calcined material and wasthen wet-pulverized by a ball mill for 16 hours, a predetermined amountof a binder was added and mixed, so that a ceramic slurry was obtained.

Next, by the same method as that of Example 2, an unfired laminate(magnetic ceramic element) including a laminate type spiral coil thereinwas formed.

In addition, this laminate was sintered by adjusting the firingtemperature so as to obtain a pore area ratio of the side gap portion of11%.

Next, in a manner similar to that of Example 2, the impedance and thebending strength by a three-point bending test were measured. Inaddition, after a heat shock test between −55° C. and 125° C. wasperformed 2,000 cycles for 50 elements per each sample, the rates ofchange in impedance before and after the test were measured, and themaximum value thereof was obtained.

Table 3 shows the values of impedances UZI), the bending strengths, andthe maximum values of the rates of change in impedance UZI) before andafter the heat shock test of the samples in which the SnO₂ additionamounts were changed.

TABLE 3 Maximum rate SnO₂ addition Impedance Bending of change in |Z|Sample amount (percent |Z| (Ω) strength by heat shock No. by weight) 100MHz (N) test (%) 14 0 681 26 14 15 0.30 669 25 11 16 0.50 660 25 7 170.75 655 25 5 18 1.00 641 24 4 19 1.25 597 22 4

As can be seen from Table 3, as the SnO₂ addition amount is increased,the rate of change in impedance before and after the heat shock test isdecreased.

However, since the bending strength and the impedance are alsodecreased, the SnO₂ addition amount is preferably set in the range of0.3 to 1.0 percent by weight.

Furthermore, as in the case of Sample Nos. 16 and 17, when the SnO₂addition amount is set in the range of 0.5 to 0.8 percent by weight, itis particularly preferable since a multilayer coil component having morestable characteristics can be obtained.

In each of the above examples, although the case in which manufacturingwas performed by a so-called sheet lamination method including a step oflaminating ceramic green sheets was described by way of example,manufacturing may also be performed by a so-called sequential printingmethod in which after a magnetic ceramic slurry and a conductive pastefor forming internal conductors are prepared, printing is performed toform a laminate having the structure as described in each of the aboveexamples.

Furthermore, manufacturing may also be performed by a so-calledsequential transfer method in which a laminate having the structure asshown in each of the above examples is formed. In this method, forexample, after a ceramic layer formed by printing (applying) a ceramicslurry on a carrier film is transferred onto a table, an electrode pastelayer formed by printing (applying) an electrode paste on a carrier filmis transferred onto the transferred ceramic layer, and the above stepsare repeatedly performed.

The multilayer coil component of the present invention may also bemanufactured by another method, and a concrete manufacturing method isnot specifically limited.

In addition, the present invention may also be applied, for example, toa multilayer inductor having an open magnetic circuit structure whichpartly contains a non-magnetic ceramic.

In addition, in each of the above examples, an acidic solution was usedas a plating solution for plating the external electrodes, and themultilayer coil component was immersed in this plating solution to cutthe bonds between the internal conductors and the magnetic ceramiclocated therearound at the interfaces therebetween; however, forexample, at the stage before the plating step is performed, themultilayer coil component may be immersed in a NiCl₂ solution (pH of 3.8to 5.4). In addition, another acidic solution may also used.

In addition, in each of the above examples, although the case in whichthe multilayer coil components were formed one by one (one-by-oneproduction case) was described by way of example, when mass productionis performed, manufacturing may be performed by a so-calledmulti-production method in which, for example, after many coil conductorpatterns are printed on surfaces of mother ceramic green sheets, and themother ceramic green sheets are laminated and pressure-bonded to eachother to form an unfired laminate block, many multilayer coil componentsare simultaneously manufactured through a step in which the laminateblock is cut in accordance with the arrangement of the coil conductorpatterns to obtain laminates for the multilayer coil components.

In addition, in each of the above examples, although the case in whichthe multilayer coil component was a multilayer impedance element wasdescribed by way of example, the present invention may also be appliedto various multilayer coil components, such as a multilayer inductor anda multilayer transformer.

Furthermore, the other points of the present invention are also notlimited to the examples described above, and the thickness of theinternal conductor, the thickness of the magnetic ceramic layer, thedimension of the product, the firing conditions of the laminate(magnetic ceramic element), and the like may be variously changed andmodified within the scope of the present invention.

As described above, according to the present invention, a highlyreliable multilayer coil component can be provided in which withoutforming voids as in the past between the magnetic ceramic layers and theinternal conductor layers which form the multilayer coil component, aninternal stress problem generated due to the difference in sinteringshrinkage behavior and coefficient of thermal expansion between themagnetic ceramic layers and the internal conductor layers can bereduced; the direct current resistance is low; and fracture of theinternal conductors caused by the surge or the like is not likely tooccur.

Hence, the present invention may be widely applied to various multilayercoil components, such as a multilayer impedance element and a multilayerinductor, each having the structure in which a coil is provided in amagnetic ceramic.

While preferred embodiments of the invention have been described above,it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the invention. The scope of the invention, therefore, isto be determined solely by the following claims.

What is claimed is:
 1. A multilayer coil component comprising: amagnetic ceramic element formed from a ceramic laminate having magneticceramic layers laminated to each other, coil-forming internal conductorsprimarily composed of Ag, the internal conductors beinginterlayer-connected to each other to form a spiral coil, and a magneticceramic disposed around and between the internal conductors withoutvoids present at interfaces between the internal conductors and themagnetic ceramic located therearound, and the internal conductors areseparated from the magnetic ceramic at the interfaces therebetween. 2.The multilayer coil component according to claim 1, further comprising aside gap portion, and wherein each of the internal conductors has a sideportion, and in the side gap portion between side portions of theinternal conductors and a corresponding side surface of the magneticceramic element, a pore area ratio of the magnetic ceramic is in therange of 6% to 20%.
 3. The multilayer coil component according to claim1, further comprising a side gap portion, wherein a pore area ratio ofthe magnetic ceramic of the side gap portion is larger than a pore arearatio of an external layer region between an upper surface of theuppermost external layer of the internal conductors in the magneticceramic element and an upper surface thereof and a pore area ratio of anexternal layer region between a lower surface of the lowermost externallayer of the internal conductors in the magnetic ceramic element and alower surface thereof.
 4. The multilayer coil component according toclaim 1, wherein the magnetic ceramic includes NiCuZn ferrite as aprimary component and contains 0.1 to 0.5 percent by weight of a zincborosilicate-based low softening point glass having a softening point of500° C. to 700° C.
 5. The multilayer coil component according to claim1, wherein the magnetic ceramic includes NiCuZn ferrite as a primarycomponent and contains 0.2 to 0.4 percent by weight of a zincborosilicate-based low softening point glass having a softening point of500° C. to 700° C.
 6. The multilayer coil component according to claim1, wherein the magnetic ceramic includes NiCuZn ferrite as a primarycomponent and contains 0.3 to 1.0 percent by weight of SnO₂ as well as0.1 to 0.5 percent by weight of a zinc borosilicate-based low softeningpoint glass having a softening point of 500° C. to 700° C.
 7. Themultilayer coil component according to claim 1, wherein the magneticceramic includes NiCuZn ferrite as a primary component and contains 0.5to 0.8 percent by weight of SnO₂ as well as 0.1 to 0.5 percent by weightof a zinc borosilicate-based low softening point glass having asoftening point of 500° C. to 700° C.
 8. The multilayer coil componentaccording to claim 2, wherein the average value of the diameters ofpores relating to the pore area ratio of the magnetic ceramic is in therange of 0.1 to 0.6 μm.
 9. The multilayer coil component according toclaim 3, wherein an average value of the diameters of pores relating tothe pore area ratio of the magnetic ceramic is in the range of 0.1 to0.6 μm.
 10. The multilayer coil component according to claim 4, whereinan average value of the diameters of pores relating to the pore arearatio of the magnetic ceramic is in the range of 0.1 to 0.6 μm.
 11. Themultilayer coil component according to claim 5, wherein an average valueof the diameters of pores relating to the pore area ratio of themagnetic ceramic is in the range of 0.1 to 0.6 μm.
 12. The multilayercoil component according to claim 6, wherein an average value of thediameters of pores relating to the pore area ratio of the magneticceramic is in the range of 0.1 to 0.6 μm.
 13. The multilayer coilcomponent according to claim 7, wherein an average value of thediameters of pores relating to the pore area ratio of the magneticceramic is in the range of 0.1 to 0.6 μm.
 14. A multilayer coilcomponent comprising: a magnetic ceramic element formed from a ceramiclaminate having magnetic ceramic layers laminated to each other,coil-forming internal conductors primarily composed of Ag, the internalconductors being interlayer-connected to each other to form a spiralcoil, and side gap portion, wherein each of the internal conductors hasa side portion, and in the side gap portion between side portions of theinternal conductors and a corresponding side surface of the magneticceramic element, a pore area ratio of the magnetic ceramic is in therange of 6% to 20%, and the internal conductors are separated from themagnetic ceramic at interfaces therebetween.
 15. The multilayer coilcomponent according to claim 14, further comprising a side gap portion,wherein a pore area ratio of the magnetic ceramic of the side gapportion is larger than a pore area ratio of an external layer regionbetween an upper surface of the uppermost external layer of the internalconductors in the magnetic ceramic element and an upper surface thereofand a pore area ratio of an external layer region between a lowersurface of the lowermost external layer of the internal conductors inthe magnetic ceramic element and a lower surface thereof.
 16. Themultilayer coil component according to claim 14, wherein the magneticceramic includes NiCuZn ferrite as a primary component and contains 0.1to 0.5 percent by weight of a zinc borosilicate-based low softeningpoint glass having a softening point of 500° C. to 700° C.
 17. Themultilayer coil component according to claim 14, wherein the magneticceramic includes NiCuZn ferrite as a primary component and contains 0.3to 1.0 percent by weight of SnO₂ as well as 0.1 to 0.5 percent by weightof a zinc borosilicate-based low softening point glass having asoftening point of 500° C. to 700° C.
 18. The multilayer coil componentaccording to claim 14, wherein the average value of the diameters ofpores relating to the pore area ratio of the magnetic ceramic is in therange of 0.1 to 0.6 μm.
 19. The multilayer coil component according toclaim 14, further comprising external electrodes formed on two sidesurfaces of the magnetic ceramic element on which two end portions ofthe spiral coil are exposed, and plating films are formed on theexternal electrodes.